High strength zirconia ceramic

ABSTRACT

A high strength zirconia ceramic composition consisting essentially of a compound of ZrO 2  containing less than 5.0 mol % Y 2  O 3  as a stabilizer and further containing 1-30 wt % aluminum and magnesium contents to the amount of the compound of ZrO 2  in terms of Al 2  O 3  and MgO. In the case that the compound of ZrO 2  further contains CeO 2  as a stabilizer, the total amount of Y 2  O 3  and CeO 2  is determined to be 1.0-15 mol % to the amount of the compound of ZrO 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to zirconia ceramics and is more particularly concerned with zirconia ceramic compositions superior in bending strength, acid resistance and thermal stability.

2. Discussion of the Background

Zirconia ceramics containing 1.5-5.0 mol% yttria as a stabilizer are each known in the art as a partially stabilized zirconia ceramic (PSZ) which has been developed as a high strength zirconia ceramic useful for mechanical structure materials. The partially stabilized zirconia ceramic is, however, still insufficient in bending strength. In Japanese Patent Early Publication No. 60-226457, there has been proposed a method producing a high strength zirconia ceramic the bending strength of which is higher than that of the partialy stabilized zirconia ceramic. In this method, raw material fine powder with a specific primary mean particle diameter was mixed at a predetermined ratio and made by Hot Isostatic Pressing or sintered by uniaxial pressing to produce a ceramic composition consisting essentially of ZrO₂ containing 1.5-5.0 mol% Y₂ O₃ and Al₂ O₃ -MgO as a stabilizer.

During change of the ambient temperature from a high temperature to room temperature, the zirconia ceramic in the cubic structure form transforms into the tetragonal structure then to the monoclinic structure form with consequent volume changes. When the crystal structure changes on cooling from tetragonal to monoclinic, the zirconia ceramic is degradated due to a volume expansion thereof. For the purpose of preventing such degradation, there has been proposed a method of restraining the transformation of crystal stucture in ZrO₂ by solid solution of a stabilizer such as CaO, MgO, Y₂ O₃ and the like. At present, Y₂ O₃ is used as the stabilizer to produce a partially stabilized zirconia ceramic of high strength and fracture toughness in the tetragonal structure form at room temperature. Such partially stabilized zirconia ceramics are, however, unstable in crystal phase and transform into the monoclinic structure when heated at a relatively low temperature within a range of 200°-400° C., resulting in deterioration in strength, fracture toughness and thermal stability thereof.

To enhance the strength and thermal stability, a zirconia ceramic composition consisting essentially of ZrO₂ containing Y₂ O₃ and Al₂ O₃ has been proposed in Japanese Patent Early Publication No. 58-32066, and a zirconia ceramic composition consisting essentially of ZrO₂ containing Y₂ O₃, CeO and Al₂ O₃ has been further proposed in Japanese Patent Early Publication No. 61-77665. In the former publication, a mixture of Al₂ O₃ and MgO was suggested, but the mixture ratio of the oxides was not confirmed. In addition, it has been found that the zirconia ceramic composition proposed in the former publication is not useful to enhance the thermal stability, whereas the zirconia ceramic composition proposed in the latter publication is not useful to enhance the strength.

SUMMARY OF THE INVENTION

The present invention has now found that the strength of partially stabilized zirconia ceramics is greatly influenced by the mixture ratio of the oxides Al₂ O₃ and MgO and that if the mixture ratio of the oxides is out of a predetermined extent, the bending strength of the partially stabilized zirconia ceramics may not be increased. It is, therefore, a primary object of the present invention to provide a partially stabilized zirconia ceramic superior in bending strength, acid resistance and thermal stability on a basis of determination of an optimal mixture ratio between the oxides.

According to the present invention, there is provided a high strength zirconia ceramic composition consisting essentially of a compound of ZrO₂ containing less than 5.0 mol% Y₂ O₃ as a stabilizer and further containing and 1-30 wt% aluminum and magnesium contents the amount of the compound of ZrO₂ in terms of Al₂ O₃ and MgO. In the case that the zirconia ceramic composition is produced without CeO₂, the ingredient amount of Y₂ O₃ has been determined to be 1.5-5.0 mol% to the compound of ZrO₂. In the case that the zirconia ceramic composition is produced with CeO₂, the ingredient amount of Y₂ O₃ has been determined to be 0.5-5.0 mol% to the compound of ZrO₂, and the ingredient amount of CeO₂ has been determined to be 0.5-12.0 mol% to the compound of ZrO₂, provided that the total amount of Y₂ O₃ and CeO₂ has been determined to be 1.0-15 mol%. It is preferable that the zirconia ceramic composition contains 1.5-5.0 mol% Y₂ O₃ (or 1.5-3.5 mol% Y₂ O₃ and 2-5 mol% CeO₂) as a stabilizer and further contains 1-5 wt% aluminum and magnesium contents to the amount of the compound of ZrO₂ in terms of Al₂ O₃ and MgO. With respect to the aluminum and magnesium contents in the composition, it is desirable that the molar ratio (Al₂ O₃ /MgO) between the aluminum and magnesium contents is determined to be in the following extents (a)-(c) in terms of Al₂ O₃ and MgO.

(a) 35-45/65 -55

(b) 60-75/40 -25

(c) 85-99/15 -1

Preparation of raw materials for the zirconia ceramic composition can be made by a method of mixing oxide powder of alumina-magnesia system or spinel powder with zirconia powder, a method of mixing each powder of alumina and magnesia with zirconia powder, a method of obtaining powder from aqueous solutions of ions of zirconium, yttrium, aluminum, magnesia and the like by means of wet mixing method. Preferably, the following two methods are adapted to preparation of the raw materials for the zirconia ceramic composition. In a primary preparation method, either mixed powder of Al₂ O₃ -MgO or aqueous salt solutions of Al₂ O₃ -MgO is added to mixed powder of ZrO₂ -Y₂ O₃ or ZrO₂ -Y₂ O₃ -CeO₂. In a secondary preparation method, either powder or aqueous salt solution of MgO is added to mixed powder of ZrO₂ -Y₂ O₃ -Al₂ O₃ or ZrO₂ -Y₂ O₃ -CeO₂ -A1₂ O₃. In these preparation methods, ZrO₂ -Y₂ O₃, ZrO₂ -Y₂ O₃ -CeO₂, ZrO₂ -Y₂ O₃ -Al₂ O₃, ZrO₂ -Y₂ O₃ -CeO₂ O₃ each may be prepared in the form of mixed powder thereof or may be prepared by calcination of mixed powders thereof. Although 0.5-3.0 wt% HfO₂ is inevitably contained in the zirconia raw material, a part of ZrO₂ may be substituted for HfO₂ in the zirconia ceramic composition of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating bending strength and acid resistance of the zirconia ceramic composition in relation to molar ratios of Al₂ O₃ and MgO in the case that the mixture ratio of aluminum and magnesium contents to the whole contents of the zirconia ceramic composition in terms of Al₂ O₃ and MgO was determined in a constant value;

FIG. 2 is a graph illustrating bending strength and acid resistance of the zirconia ceramic composition in relation to weight ratios of Al₂ O₃ and MgO to the whole contents of the zirconia ceramic composition in the case that the molar ratio between aluminum and magnesium contents was determined in a constant value;

FIG. 3 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (mol%) of Y₂ O₃ in the composition;

FIG. 4 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (mol%) of CeO₂ in the composition;

FIG. 5 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (wt%) of aluminum and magnesium contents in terms of Al₂ O₃ and MgO in the composition;

FIG. 6 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a molar ratio of Al₂ O₃ and MgO in the composition; and

FIG. 7 is a graph illustrating the rate of transformation from cubic structure into tetragonal structure and to monoclinic structure in relation to a mixed amount (mol%) of CeO₂ and Y₂ O₃ in the zirconia ceramic composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

A compound of ZrO₂ -Y₂ O₃ was precipitated by hydrolysis of aqueous salt solutions thereof. The precipitated compound was calcined at 900° C. to obtain zirconia powder of particle diameter less than 1 μm. The zirconia powder was added with powder of Al₂ O₃ and MgO, ground by a pot mill and dried by spray to obtain a raw material consisting essentially of ZrO₂ -Y₂ O₃ -Al₂ O₃ -MgO.

The raw material was preformed under pressure of 200 kg/cm² and shaped by Cold Isostatic Pressing into a rectangular plate of 60×60×8 (mm) under pressure of 3 ton/cm². The rectangular plate was fired by pressureless sintering at about 1400° C. for 5 hours and cut into a plurality of bar specimens each size of which is 3×4××40 (mm). Thus, the bar specimens were tested as follows.

(1) The bending strengths were measured by 4-point bending test with 10mm upper span and 30mm lower span at a cross-head speed 0.5mm/min using the bar specimens.

(2) For measurement of the acid resistances, the bar specimens were put into solution of 36 wt% HCl in a sealed container and retained in the solution at 150° C. for 200hours. Thereafter, the weight of bar specimens was measured to calculate reduction of the weight per a unit area (mg/cm²).

The test results of the bar specimens are shown in Tables 1 and 2 attached hereto and illustrated in FIGS. 1 and 2. The test results of No. 1-No. 27 shown in Tables 1 and 2 was obtained in the case that a molar ratio between Al₂ O₃ and MgO or an added amount of Al₂ O₃ and MgO was varied in a condition where the molar ratio of a stabilizer Y₂ O₃ to the compound ZrO₂ was determined in a constant value (3/97). The data of FIG. 1 was obtained by variation of the molar ratio between Al₂ O₃ and MgO in a condition where the added amount of Al₂ O₃ and MgO was determined in a constant value (2 wt%). The data of FIG. 2 was obtained by variation of the added amount of Al₂ O₃ and MgO in a condition where the molar ratio between Al₂ O₃ and MgO was determined in a constant value (40/60, 70/30, 90/10). In FIGS. 1 and 2, a dot-dash straight line indicates the bending strength and acid resistance of a bar specimen prepared without Al₂ O₃ -MgO. As is illustrated in FIGS. 1 and 2, the acid resistance of the bar specimens added with Al₂ O₃ and MgO was noticeably enhanced in the case that the molar ratio between Al₂ O₃ and MgO was determined more than 30/60, and the bending strength of the bar specimens was increased up to a peak value in the case that the molar ratio between Al₂ O₃ and MgO was determined to be 40/60, 70/30 and 90/10, respectively. Particularly, the bending strength of the bar specimens was noticeably increased in the case that the molar ratio between Al₂ O₃ and MgO was determined to be 35-45/65-55, 60-75/40-25, 85-99/15-1, respectively. In addition, the bending strength was increased in the case that the added amount of Al₂ O₃ and MgO was determined to be less than 20 wt%, preferably less than 10 wt%.

The test results of No. 28-No. 43 shown in Table 2 were obtained in the case that the molar ratio between Al₂ O₃ and MgO was varied in a condition where the molar ratio of a stabilizer Y₂ O₃ to the compound ZrO₂ was determined in a constant value (1.5/98.5, 2/98, 5/95) and where the added amount of Al₂ O₃ and MgO was determined in a constant value (2 wt%, 5 wt%). The test results of No. 28-No. 43 indicate that the bending strength and acid resistance of the bar specimens were enhanced in contrast with the bar specimen prepared without Al₂ O₃ and MgO.

From the test results described above, it will be understood that the zirconia cermaic composition of the present invention is superior in bending strength and acid resistance and useful as mechanical structure materials. In example 1, Al₂ O₃ and MgO were adapted to contain aluminum and magnesium contents into the compound of ZrO₂ -Y₂ O₃. The amount of the aluminum and magnesium contents is consistent with an amount of aluminum and magnesium contents in a ceramic composition ground in 44 μm after firing and measured by fluorescent X-ray analysis. For this reason, the amount of aluminum and magnesium contents in terms of Al₂ O₃ and MgO is substantially consistent with the added amount of Al₂ O₃ and MgO. In addition, it was found by fluorescent X-ray analysis that the following impurity is contained in the bar specimens:

less than 2.0 wt% SiO₂, less than 2.0 wt% TiO₂, less than 0.5 wt% CaO, less than 0.5 wt% K₂ O, less than 0.5 wt% Na₂ O and less than 3.0 wt% HfO₂.

EXAMPLE 2

A compound of ZrO₂ -Y₂ O₃ -CeO₂ was precipitated by hydrolysis of aqueous salt solutions thereof. The precipitated compound was calcined at 900° C. to obtain zirconia powder of particle diameter less than 1 μm. The zirconia powder was added with powder of Al₂ O₃ and MgO, ground by a port mill and dried by spray to obtain a raw material consisting essentially of ZrO₂ -Y₂ O₃ -CeO₂ -Al₂ O₃ -MgO.

The raw material was preformed under pressure of 200 kg/cm² and shaped by Cold Isostatic Pressing into a rectangular plate of 60×60×8 (mm) under pressure of 3 ton/cm². The rectangular plate was fired by pressureless sintering at about 1400° C. for 5 hours and cut into a plurality of bar specimens each size of which is 3×4×40 (mm). Thus, the bar specimens were tested as follows.

(1) The bending strengths were measured by 4-point bending test with 10mm upper span and 30mm lower span at a cross-head speed 0.5mm/min using the bar specimens.

(2) For measurement of the thermal deterioration, the bar specimens were put into hot water at 250° C. under vapor pressure of 39 kg/cm² in an autoclave and treated by heat for 50 hours. Thereafter, the rate of transformation from cubic structure into tetragonal structure and to monoclinic structure in the bar specimens were calculated as follows.

The bar specimens were mirror surface finished with diamond paste and applied to X-ray diffraction to measure integration intensity IM, IT, IC of diffraction peaks on the monoclinic crystal surface (111), the tetragonal crystal surface (111), and the cubic crystal surface thereby to calculate an amount of tetragonal and cubic contents V₀ =(IT+IC)/(IM+IT+IC). The bar specimens after heat treatment were further applied to X-ray diffraction to calculate an amount of tetragonal and cubic contents (V₁) in the same manner as described above. Thus, the rate of transformation (%)=(V₀ -V₁)/V₀ ×100 was calculated on a basis of the calculated amounts of tetragonal and cubic contents V₀ and V₁.

The test results of the bar specimens are shown in Tables 3-11 attached hereto and illustrated in FIGS. 3-7. In the test results, the value of 70 kgf/mm² was defined as a standard value for determination of the bending strength, and the value of 25% was defined as a standard value for determination of the thermal stability. In FIG. 3, the specimens in eight groups A-H are plotted near or below the standard value 70 kgf/mm². The specimens in groups A-D were prepared to contain less than 82-85 mol% ZrO₂ and more than 15 mol% Y₂ O₃ and CeO₂, the specimens in groups E and F to contain 85 mol% ZrO₂ and 15 mol% Y₂ O₃ and CeO₂, the specimens in group G to contain 5 mol% Y₂ O₃, and the specimens to group H to contain 7 mol% Y₂ O₃. In FIG. 4 there is illustrated a relationship between a mixed amount of CeO₂ (mol%) in a zirconia ceramic composition containing 3 mol% Y₂ O₃ and 2 wt% Al₂ O₃ and MgO and bending strength of the same ceramic composition. In this illustration, it was found that the bending strengths are decreased in accordance with an increase of the mixed amount of CeO₂ and noticeably decreased by mixture of CeO₂ more than 12 mol%. From the test results described above, it was confirmed that a high strength zirconia ceramic composition can be obtained by a compound of ZrO₂ containing 0.5-5 mol% Y₂ O₃, 0.5-12 mol% CeO₂, Al₂ O₃ and MgO, provided that the total amount of Y₂ O₃ and CeO₂ should be determined to be 1.0-15 mol%.

In FIG. 5 there is illustrated a relationship between a mixed amount (wt%) of Al₂ O₃ and MgO in a zirconia ceramic composition containing 3 mol% Y₂ O₃ and 2 mol% CeO₂ and bending strength of the same ceramic composition. In this case, the mixed amount (wt%) of aluminum and magnesium contents was determined in terms of Al₂ O₃ and MgO. In this illustration, it was found that the bending strengths are decreased in accordance with an increase of the mixed amount of Al₂ O₃ and MgO and noticeably decreased by mixture of Al₂ O₃ and MgO more than 30 wt%. In FIG. 6 there is illustrated a relationship between a molar ratio Al₂ O₃ /MgO in a zirconia ceramic composition containing 3 mol% Y₂ O₃, 2 mol% CeO₂ and 2 wt% Al₂ O₃ and MgO and bending strength of the same ceramic composition. In this illustration, it was found that each peak of the bending strengths is obtained by determination of the molar ratio Al₂ O₃ /MgO is 40/60, 70/30 or 90/10. Preferably, the molar ratio Al₂ O₃ /MgO should be determined in a extent of 35-45/65-55, 60-75/40-25 or 85-99/15-1. From the test results, it was confirmed that a high strength zirconia ceramic composition can be obtained by a compound a ZrO₂ containing less than 30 wt% Al₂ O₃ and MgO at the molar ratio listed below.

(a) 35-45/65-55

(b) 60-75/40-25

(c) 85-99/15-1

As is understood from the data of FIGS. 3-5, it was confirmed that the bending strength is noticeably increased in a zirconia ceramic composition containing 2 mol% Y₂ O₃, less than 5 mol% CeO₂, 1-5 mol% Al₂ O₃ a MgO. In FIG. 7 there is illustrated a relationship between mixed amounts (mol%) of Y₂ O₃ and CeO₂ in a zirconia ceramic composition containing 2.0 wt% at a molar ratio 90/10 and the rate of transformation of the tetragonal structure and cubic structure in the ceramic composition. From the data of FIG. 7, it was confirmed that the rate of transformation is decreased in accordance with an increase of the mixed amounts of Y₂ O₃ and CeO₂. This means that the thermal stability of the zirconia ceramic composition is greatly enhanced by an increase of the mixed amounts of Y₂ O₃ and CeO₂.

EXAMPLE 3

Powders of ZrO₂ -Y₂ O₃ and ZrO₂ -Y₂ O₃ -CeO₂ were prepared in the same manner as described in Example 1 added with aluminum and magnesium contents in the form of Al(OH)₃, AlCl₃, Al(NO₃)₃, Al₂ (C₂ O₄)₃ Mg(OH)₂, MgCl₂, Mg(NO₃)₂, MgC₂ O₄, MgCO₃ or the like, ground by a pot mill and dried by spray to obtain a raw material consisting essentially of ZrO₂ -Y₂ O₃ and the aluminum and magnesium contents and a raw material consisting essentially of ZrO₂ -Y₂ O₃ -CeO₂ and the aluminum and magnesium contents.

Using the raw materials, various bar specimens were prepared in the same manner as described in Example 1 and tested in the same manner as described in Example 1. The test results of the bar specimens are shown in Tables 12-15 attached hereto. From the data of Tables 12-15, it was confirmed that the use of aluminum and magnesium contents in the form of the above-described compounds is effective to obtain the same results as those in use of Al₂ O₃ -MgO.

EXAMPLE 4

(1) Sol-solution was prepared by hydrolysis of aqueous solution containing zirconium oxychloride added with yttrium chloride or yttrium chloride and cerium chloride. The sol-solution was added with Al(OH)₃ and Mg(OH)₂, treated by heat, ground and dried in the same manner as described in Example 1 to obtain a raw material.

(2) The sol-solution was added with Al(OH)₃ in such a manner that a molar ratio to MgO is determined to be 90/10 in terms of Al₂ O₃. Powder obtained by heat treatment of the sol-solution was added with powder of MgO, ground and dried in the same manner as described in Example 1 to obtain a raw material.

Using the raw materials, various bar specimens were prepared in the same manner as described in Example 1 and tested in the same manner as described in Example 1. The test results of the bar specimens are shown in Table 16 attached hereto. From the data of Table 16, it was confirmed that the use of zirconium, yttrium and cerium ions in the form of aqueous salt solution is effective to obtain the same results as those in use of precipitated ZrO₂ -Y₂ O₃ or ZrO₂ -Y₂ O₃ -CeO₂.

                  TABLE 1                                                          ______________________________________                                                   Al.sub.2 O.sub.3 /MgO                                                                     Bending   Acid                                                 Molar Ratio                                                                               Molar   Amount Strength                                                                               Resistance                              No   (Y.sub.2 O.sub.3 /ZrO.sub.2)                                                              Ratio   (wt %) (kgf/mm.sup.2)                                                                         (mg/cm.sup.2)                           ______________________________________                                         1    3/97       20/80   2      52.0    36.4                                    2    3/97       30/70   2      71.2    27.5                                    3    3.97       40/60   2      99.4    3.0                                     4    3/97       50/50   2      69.2    3.6                                     5    3/97       55/45   2      70.4    4.0                                     6    3/97       70/30   2      105.2   3.1                                     7    3/97       80/20   2      72.3    4.5                                     8    3/97       90/10   2      110.5   3.5                                     9    3/97       100/0   2      50.1    3.2                                     10   3/97       40/60   1      98.7    4.5                                     11   3/97       40/60   5      98.2    2.7                                     12   3/97       40/60   10     90.5    2.7                                     13   3/97       40/60   20     87.8    2.6                                     14   3/97       40/60   30     84.5    2.5                                     15   3/97       70/30   1      99.5    4.5                                     16   3/97       70/30   5      103.0   3.0                                     17   3/97       70/30   10     95.2    2.9                                     18   3/97       70/30   20     89.8    2.9                                     19   3/97       70/30   30     84.8    2.6                                     20   3/97       90/10   1      99.2    5.1                                     21   3/97       90/10   5      102.0   3.1                                     22   3/97       90/10   10     95.4    3.1                                     ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                   Al.sub.2 O.sub.3 /MgO                                                                     Bending   Acid                                                 Molar Ratio                                                                               Molar   Amount Strength                                                                               Resistance                              No   (Y.sub.2 O.sub.3 /ZrO.sub.2)                                                              Ratio   (wt %) (kgf/mm.sup.2)                                                                         (mg/cm.sup.2)                           ______________________________________                                         23   3/97       90/10   20     90.2    2.9                                     24   3/97       90/10   30     85.2    2.7                                     25   3/97       30/70   5      67.7    28.1                                    26   3/97       50/50   20     51.3    3.7                                     27   3/97       --      0      92.0    73.1                                    28   1.5/98.5   40/60   2      78.2    52.1                                    29   1.5/98.5   70/30   2      78.2    45.0                                    30   1.5/98.5   90/10   2      77.2    41.0                                    31   1.5/98.5   --      0      62.0    120.2                                   32   2/98       40/60   2      98.7    37.0                                    33   2/98       70/30   2      98.5    37.0                                    34   2/98       90/10   2      100.0   36.8                                    35   2/98       100/0   2      52.1    32.0                                    36   2/98       40/60   5      99.0    35.0                                    37   2/98       70/30   5      102.1   36.5                                    38   2/98       90/10   5      106.0   36.2                                    39   2/98       --      0      90.5    92.1                                    40   5/95       40/60   2      79.2    32.0                                    41   5/95       70/30   2      78.8    29.0                                    42   5/95       90/10   2      79.5    38.7                                    43   5/95       --      0      58.5    122.4                                   ______________________________________                                    

                                      TABLE 3                                      __________________________________________________________________________     Composition                                                                                     Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                   No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                    __________________________________________________________________________      1                                                                               99.5 0.5  0    40/60  2    --    Degradated                                   2                                                                               89.5 "    10.0 "      "    75.5  21                                           3                                                                               85.0 "    14.5 "      "    84.5  7                                            4                                                                               82.5 "    17.0 "      "    45.0  4                                            5                                                                               99.5 "    0    70/30  "    --    Degradated                                   6                                                                               89.5 "    10.0 "      "    74.8  18                                           7                                                                               85.0 "    14.5 "      "    85.3  7                                            8                                                                               82.0 "    17.0 "      "    52.1  4                                            9                                                                               99.5 "    0    90/10  "    --    Degradated                                  10                                                                               89.5 "    10.0 "      "    78.3  18                                          11                                                                               85.0 "    14.5 "      "    87.5  7                                           12                                                                               82.5 "    17.0 "      "    49.8  3                                           13                                                                               98.0 2.0  0    40/60  "    98.7  Degradated                                  14                                                                               95.0 "    3.0  "      "    92.5  30                                          15                                                                               93.0 "    5.0  "      "    87.0  15                                          16                                                                               85.0 "    13.0 "      "    74.0  5                                           17                                                                               82.0 "    16.0 "      "    47.3  4                                           18                                                                               98.0 "    0    70/30  "    98.5  Degradated                                  19                                                                               95.0 "    3.0  "      "    93.7  27                                          20                                                                               93.0 "    5.0  "      "    97.5  15                                          21                                                                               85.0 "    13.0 "      "    72.1  4                                           22                                                                               82.0 "    16.0 "      "    48.5  4                                           __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________     Composition                                                                                     Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                   No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                    __________________________________________________________________________     23                                                                               98.0 2.0  0    90/10  2    106.0                                             24                                                                               95.0 "    3.0  "      "    97.3  28                                          25                                                                               93.0 "    5.0  "      "    107.0 13                                          26                                                                               85.0 "    13.0 "      "    78.3  5                                           27                                                                               82.0 "    16.0 "      "    52.1  4                                           28                                                                               97.0 3.0  0    40/60  "    98.7  39                                          29                                                                               96.5 "    0.5  "      "    94.2  31                                          30                                                                               96.0 "    1.0  "      "    94.5  22                                          31                                                                               95.0 "    2.0  "      "    92.1  17                                          32                                                                               94.0 "    3.0  "      "    89.9  14                                          33                                                                               92.0 "    5.0  "      "    87.4  8                                           34                                                                               87.0 "    10.0 "      "    73.1  4                                           35                                                                               85.0 "    12.0 "      "    71.1  4                                           36                                                                               83.0 "    14.0 "      "    47.0  4                                           37                                                                               97.0 "    0    70/30  "    102.1 37                                          38                                                                               96.5 "    0.5  "      "    98.5  30                                          39                                                                               96.0 "    1.0  "      "    97.8  24                                          40                                                                               95.0 "    2.0  "      "    97.8  18                                          41                                                                               94.0 "    3.0  "      "    95.3  15                                          42                                                                               92.0 "    5.0  "      "    89.0  8                                           43                                                                               87.0 "    10.0 "      "    75.2  5                                           __________________________________________________________________________

                                      TABLE 5                                      __________________________________________________________________________     Composition                                                                                     Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                   No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                    __________________________________________________________________________     44                                                                               85.0 3.0  12.0 70/30  2    70.1  4                                           45                                                                               83.0 "    14.0 "      "    53.4  4                                           46                                                                               97.0 "    0    90/10  "    115.0 39                                          47                                                                               96.5 "    0.5  "      "    110.2 27                                          48                                                                               96.0 "    1.0  "      "    100.5 25                                          49                                                                               95.0 "    2.0  "      "    109.4 21                                          50                                                                               94.0 "    3.0  "      "    98.7  17                                          51                                                                               92.0 "    5.0  "      "    97.2  10                                          52                                                                               87.0 "    10.0 "      "    80.5  5                                           53                                                                               85.0 "    12.0 "      "    72.1  5                                           54                                                                               83.0 "    14.0 "      "    57.2  4                                           55                                                                               94.5 5.0  0.5  40/60  "    73.2  12                                          56                                                                               93.0 "    2.0  "      "    70.5  3                                           57                                                                               94.5 "    0.5  70/30  "    74.2  11                                          58                                                                               93.0 "    2.0  "      "    74.7  3                                           59                                                                               94.5 "    0.5  90/10  "    76.2  10                                          60                                                                               93.0 "    2.0  "      "    75.3  3                                           61                                                                               92.5 7.0  0.5  40/60  "    56.2  7                                           62                                                                               91.0 "    2.0  "      "    44.5  3                                           63                                                                               92.5 "    0.5  70/30  "    55.3  7                                           64                                                                               91.0 "    2.0  "      "    45.5  3                                           65                                                                               92.5 "    0.5  90/10  "    56.4  7                                           __________________________________________________________________________

                                      TABLE 6                                      __________________________________________________________________________     Composition                                                                                     Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                   No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                    __________________________________________________________________________     66                                                                               91.0 7.0  2.0  90/10  2    48.2  3                                           67                                                                               97.0 3.0  0    40/60  1    98.7  42                                          68                                                                               95.0 "    2.0  "      "    92.8  25                                          69                                                                               94.0 "    3.0  "      "    95.2  19                                          70                                                                               92.0 "    5.0  "      "    87.1  9                                           71                                                                               85.0 "    12.0 "      "    72.3  5                                           72                                                                               83.0 "    14.0 "      "    54.2  4                                           73                                                                               97.0 "    0    70/30  "    99.5  41                                          74                                                                               95.0 "    2.0  "      "    92.1  24                                          75                                                                               94.0 "    3.0  "      "    92.4  19                                          76                                                                               92.0 "    5.0  "      "    91.2  7                                           77                                                                               85.0 "    12.0 "      "    71.5  5                                           78                                                                               83.0 "    14.0 "      "    53.1  5                                           79                                                                               97.0 "    0    90/10  "    99.2  39                                          80                                                                               95.0 "    2.0  "      "    96.5  24                                          81                                                                               94.0 "    3.0  "      "    93.7  15                                          82                                                                               92.0 "    5.0  "      "    89.0  7                                           83                                                                               85.0 "    12.0 "      "    73.2  5                                           84                                                                               83.0 "    14.0 "      "    58.5  5                                           85                                                                               97.0 "    0    40/60  5    98.2  32                                          86                                                                               95.0 "    2.0  "      "    95.6  20                                          __________________________________________________________________________

                                      TABLE 7                                      __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                  No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                   __________________________________________________________________________      87                                                                               94.0 3.0  3.0  40/60   5   93.4  11                                          88                                                                               92.0 "    5.0  "      "    85.1  9                                           89                                                                               85.0 "    12.0 "      "    70.2  5                                           90                                                                               83.0 "    14.0 "      "    60.0  4                                           91                                                                               97.0 "    0    70/30  "    103.0 30                                          92                                                                               95.0 "    2.0  "      "    97.3  23                                          93                                                                               94.0 "    3.0  "      "    95.2  10                                          94                                                                               92.0 "    5.0  "      "    90.0  8                                           95                                                                               85.0 "    12.0 "      "    74.1  5                                           96                                                                               83.0 "    14.0 "      "    58.0  4                                           97                                                                               97.0 "    0    90/10  "    102.0 32                                          98                                                                               95.0 "    2.0  "      "    100.3 23                                          99                                                                               94.0 "    3.0  "      "    98.6  12                                         100                                                                               92.0 "    5.0  "      "    89.4  8                                          101                                                                               85.0 "    12.0 "      "    78.5  5                                          102                                                                               83.0 "    14.0 "      "    61.0  4                                          103                                                                               97.0 "    0    40/60  10   90.5  25                                         104                                                                               95.0 "    2.0  "      "    88.2  17                                         105                                                                               92.0 "    5.0  "      "    72.1  5                                          106                                                                               85.0 "    12.0 "      "    67.5  4                                          107                                                                               83.0 "    14.0 "      "    44.3  4                                          108                                                                               97.0 "    0    70/30  "    95.2  25                                         109                                                                               95.0 "    2.0  "      "    89.5  14                                         __________________________________________________________________________

                                      TABLE 8                                      __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                  No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                   __________________________________________________________________________     110                                                                               92.0 3.0  5.0  70/30  10   72.1  4                                          111                                                                               85.0 "    12.0 "      "    65.5  4                                          112                                                                               83.0 "    14.0 "      "    50.9  3                                          113                                                                               97.0 "    0    90/10  "    95.4  24                                         114                                                                               95.0 "    2    "      "    91.3  12                                         115                                                                               92.0 "    5.0  "      "    80.2  6                                          116                                                                               85.0 "    12.0 "      "    74.4  4                                          117                                                                               83.0 "    14.0 "      "    57.5  4                                          118                                                                               97.0 "    0    40/60  20   87.8  20                                         119                                                                               95.0 "    2.0  "      "    84.5  12                                         120                                                                               92.0 "    5.0  "      "    74.7  5                                          121                                                                               85.0 "    12.0 "      "    58.2  3                                          122                                                                               83.0 "    14.0 "      "    60.0  3                                          123                                                                               97.0 "    0    70/30  "    89.8  21                                         124                                                                               95.0 "    2.0  "      "    89.2  14                                         125                                                                               92.0 "    5.0  "      "    72.2  4                                          126                                                                               85.0 "    12.0 "      "    60.3  3                                          127                                                                               83.0 "    14.0 "      "    54.1  4                                          128                                                                               97.0 "    0    90/10  "    90.2  18                                         129                                                                               95.0 "    2.0  "      "    87.2  10                                         130                                                                               92.0 "    5.0  "      "    74.2  5                                          131                                                                               85.0 "    12.0 "      "    57.6  4                                          __________________________________________________________________________

                                      TABLE 9                                      __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                  No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                   __________________________________________________________________________     132                                                                               83.0 3.0  14.0 90/10  20   56.1  4                                          133                                                                               97.0 "    0    40/60  30   84.5  17                                         134                                                                               95.0 "    2.0  "      "    82.1  12                                         135                                                                               92.0 "    5.0  "      "    69.2  5                                          136                                                                               85.0 "    12.0 "      "    62.1  3                                          137                                                                               83.0 "    14.0 "      "    49.5  3                                          138                                                                               97.0 "    0    70/30  "    84.8  18                                         139                                                                               95.0 "    2.0  "      "    82.1  14                                         140                                                                               92.0 "    5.0  "      "    70.3  3                                          141                                                                               85.0 "    12.0 "      "    64.3  3                                          142                                                                               83.0 "    14.0 "      "    51.2  3                                          143                                                                               97.0 "    0    90/10  "    86.4  18                                         144                                                                               95.0 "    2.0  "      "    84.2  9                                          145                                                                               92.0 "    5.0  "      "    74.7  4                                          146                                                                               85.0 "    12.0 "      "    67.4  3                                          147                                                                               83.0 "    14.0 "      "    52.5  3                                          148                                                                               97.0 "    0    40/60  40   72.2  15                                         149                                                                               95.0 "    2.0  "      "    67.2  10                                         150                                                                               92.0 "    5.0  "      "    56.8  5                                          151                                                                               85.0 "    12.0 "      "    55.0  4                                          152                                                                               83.0 "    14.0 "      "    47.0  3                                          153                                                                               97.0 "    0    70/30  "    73.4  13                                         __________________________________________________________________________

                                      TABLE 10                                     __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                  No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                   __________________________________________________________________________     154                                                                               95.0 3.0  2.0  70/30  40   68.5  10                                         155                                                                               92.0 "    5.0  "      "    58.2  5                                          156                                                                               85.0 "    12.0 "      "    54.2  3                                          157                                                                               83.0 "    14.0 "      "    48.7  3                                          158                                                                               97.0 "    0    90/10  "    76.7  16                                         159                                                                               95.0 "    2.0  "      "    74.2  12                                         160                                                                               92.0 "    5.0  "      "    65.2  5                                          161                                                                               85.0 "    12.0 "      "    58.5  3                                          162                                                                               83.0 "    14.0 "      "    49.2  3                                          163                                                                               95.0 "    2.0  30/70  2.0  67.5  32                                         164                                                                               "    "    "    35/65  "    86.8  24                                         165                                                                               "    "    "    45/55  "    80.7  13                                         166                                                                               "    "    "    50/50  "    65.8  10                                         167                                                                               "    "    "    60/40  "    88.7  12                                         168                                                                               "    "    "    75/25  "    81.2  12                                         169                                                                               "    "    "    80/20  "    68.2  12                                         170                                                                               "    "    "    85/15  "    84.9  14                                         171                                                                               "    "    "    99/1   "    89.5  10                                         172                                                                               "    "    "    100/0  "    77.0  10                                         173                                                                               85.0 0.5  10.0 --      0   57.2  25                                         174                                                                               93.5 2.0  5.0  --     "    72.0  13                                         __________________________________________________________________________

                                      TABLE 11                                     __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Trans-                                        ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             formation                                  No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       Rate (%)                                   __________________________________________________________________________     175                                                                               95.0 3.0  2.0  --     0    68.7  23                                         176                                                                               94.0 5.0  1.0  --     "    62.1  4                                          __________________________________________________________________________

                                      TABLE 12                                     __________________________________________________________________________     Composition                                      Acid                                                         Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Resis-                        ZrO.sub.2                                                                             Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Addition      Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             tance                         No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             Al compound                                                                           Mg compound                                                                           (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       (mg/cm.sup.2)                 __________________________________________________________________________      1                                                                               97.0 3.0  0    Al(OH).sub.3                                                                          Mg(OH).sub.2                                                                          40/60  2.0  100.5 3.2                            2                                                                               "    "    "    "      "      "      5.0  105.1 2.5                            3                                                                               "    "    "    "      "      "      20.0 92.1  2.1                            4                                                                               "    "    "    "      "      "      30.0 85.2  2.5                            5                                                                               "    "    "    "      "      "      40.0 65.2  2.9                            6                                                                               "    "    "    "      "      70/30  2.0  105.2 3.0                            7                                                                               "    "    "    "      "      "      5.0  109.2 2.1                            8                                                                               "    "    "    "      "      "      20.0 93.1  2.1                            9                                                                               "    "    "    "      "      "      30.0 87.8  2.5                           10                                                                               "    "    "    "      "      "      40.0 68.3  3.2                           11                                                                               "    "    "    "      "      90/10  2.0  125.0 2.5                           12                                                                               "    "    "    "      "      "      5.0  115.0 3.1                           13                                                                               "    "    "    "      "      "      20.0 95.0  3.1                           14                                                                               "    "    "    "      "      "      30.0 90.0  2.5                           15                                                                               "    "    "    "      "      "      40.0 75.7  2.8                           16                                                                               95.0 5.0  "    AlCl.sub.3                                                                            MgCl.sub.2                                                                            40/60  2.0  78.2  30.0                          17                                                                               "    "    "    "      "      70/30  "    76.5  25.3                          18                                                                               "    "    "    "      "      90/10  "    78.2  27.5                          19                                                                               97.0 3.0  "    "      "      40/60  "    98.5  3.2                           20                                                                               "    "    "    "      "      "      5.0  97.9  3.0                           21                                                                               "    "    "    "      "      70/30  2.0  110.5 3.4                           22                                                                               "    "    "    "      "      "      5.0  109.5 2.2                           __________________________________________________________________________

                                      TABLE 13                                     __________________________________________________________________________     Composition                                      Acid                                                         Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Resis-                        ZrO.sub.2                                                                             Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Addition      Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             tance                         No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             Al compound                                                                           Mg compound                                                                           (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       (mg/cm.sup.2)                 __________________________________________________________________________     23                                                                               97.0 3.0  0    AlCl.sub.3                                                                            MgCl.sub.2                                                                            90/10  2.0  119.2 2.5                           24                                                                               "    "    "    "      "      "      5.0  108.2 2.5                           25                                                                               "    "    "    Al(OH).sub.3                                                                          "      40/60  2.0  107.2 2.8                           26                                                                               "    "    "    "      "      70/30  "    118.5 2.5                           27                                                                               "    "    "    "      "      90/10  "    119.5 2.5                           28                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    Mg(NO.sub.3).sub.2                                                                    "      "    108.2 2.1                           29                                                                               "    "    "    Al.sub.2 (C.sub.2 O.sub.4).sub. 3                                                     MgC.sub.2 O.sub.4                                                                     "      "    122.7 2.2                           30                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    MgC.sub.2 O.sub.4                                                                     "      "    105.6 2.5                           31                                                                               "    "    "    Al(OH).sub.3                                                                          MgCO.sub.3                                                                            "      "                                        32                                                                               "    "    "    "      Mg(OH).sub.2                                                                          30/70  "    72.2  32.1                          33                                                                               "    "    "    "      "      50/50  "    65.3  3.6                           34                                                                               "    "    "    "      "      80/20  "    60.2  3.2                           35                                                                               "    "    "    "      --     100/0  "    72.3  4.5                           36                                                                               "    "    "    AlCl.sub.3                                                                            MgCl.sub.2                                                                            30/70  "    68.5  40.3                          37                                                                               "    "    "    "      "      50/50  "    62.1  4.2                           38                                                                               "    "    "    "      "      80/20  "    63.4  3.2                           39                                                                               "    "    "    "      --     100/0  "    68.5  3.5                           40                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    Mg(NO.sub.3).sub.2                                                                    50/50  "    65.1  4.2                           41                                                                               "    "    "    Al(OH).sub.3                                                                          MgCO.sub.3                                                                            "      "    64.2  3.5                           42                                                                               95.0 "      2.0                                                                               Al(OH).sub.3                                                                          Mg(OH).sub.2                                                                          40/60  "    93.2  --                            __________________________________________________________________________

                                      TABLE 14                                     __________________________________________________________________________     Composition                                                                                                   Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Acid                          ZrO.sub.2                                                                             Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Addition      Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             Resistance                    No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             Al compound                                                                           Mg compound                                                                           (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       (mg/cm.sup.2)                 __________________________________________________________________________     43                                                                               95.0 3.0  2.0  Al(OH).sub.3                                                                          Mg(OH).sub.2                                                                          40/60  5.0  92.1  15                            44                                                                               "    "    "    "      "      "      20.0 86.5  10                            45                                                                               "    "    "    "      "      "      30.0 87.1  12                            46                                                                               "    "    "    "      "      "      40.0 58.8  4                             47                                                                               "    "    "    "      "      70/30  2.0  101.2 35                            48                                                                               "    "    "    "      "      "      5.0  97.5  25                            49                                                                               "    "    "    "      "      "      20.0 92.1  10                            50                                                                               "    "    "    "      "      "      30.0 83.2  10                            51                                                                               "    "    "    "      "      "      40.0 60.2  5                             52                                                                               "    "    "    "      "      90/10  2.0  112.1 21                            53                                                                               "    "    "    "      "      "      5.0  105.2 17                            54                                                                               "    "    "    "      "      "      20.0 92.7  12                            55                                                                               "    "    "    "      "      "      30.0 84.0  10                            56                                                                               "    "    "    "      "      "      40.0 64.8  6                             57                                                                               92.0 "    5.0  "      "      40/60  2.0  89.5  8                             58                                                                               "    "    "    "      "      70/30  "    92.1  5                             59                                                                               "    "    "    "      "      90/10  "    96.5  8                             60                                                                               95.0 2.0  "    AlCl.sub.3                                                                            MgCl.sub.2                                                                            40/60  2.0  90.5  15                            61                                                                               "    "    "    "      "      "      5.0  89.3  15                            62                                                                               "    "    "    "      "      70/30  2.0  99.8  14                            63                                                                               "    "    "    "      "      "      5.0  98.2  18                            __________________________________________________________________________

                                      TABLE 15                                     __________________________________________________________________________     Composition                                                                                                   Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Acid                          ZrO.sub.2                                                                             Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Addition      Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             Resistance                    No                                                                               (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             Al compound                                                                           Mg compound                                                                           (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       (mg/cm.sup.2)                 __________________________________________________________________________     64                                                                               95.0 3.0  2.0  AlCl.sub.3                                                                            MgCl.sub.2                                                                            90/10  2.0  108.2 21                            65                                                                               "    "    "    "      "      "      5.0  103.1 18                            66                                                                               "    "    "    Al(OH).sub.3                                                                          "      60/40  2.0  91.2  18                            67                                                                               "    "    "    "      "      70/30  "    96.5  15                            68                                                                               "    "    "    "      "      90/10  "    105.0 25                            69                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    Mg(NO.sub.3).sub.2                                                                    "      "    98.4  18                            70                                                                               "    "    "    Al.sub.2 (C.sub.2 O.sub.4).sub.3                                                      MgC.sub. 2 O.sub.4                                                                    "      "    110.3 15                            71                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    MgC.sub.2 O.sub.4                                                                     "      "    108.2 15                            72                                                                               "    "    "    Al(OH).sub.3                                                                          MgCO.sub.3                                                                            "      "    98.5  20                            73                                                                               "    "    "    "      Mg(OH).sub.2                                                                          30/70  "    65.2  37                            74                                                                               "    "    "    "      "      50/50  "    62.1  10                            75                                                                               "    "    "    "      "      80/20  "    65.3  12                            76                                                                               "    "    "    "      --     100/0  "    69.1  9                             77                                                                               "    "    "    AlCl.sub.3                                                                            MgCl.sub.2                                                                            30/70  "    64.4  34                            78                                                                               "    "    "    "      "      50/50  "    67.1  15                            79                                                                               "    "    "    "      "      80/20  "    63.7  12                            80                                                                               "    "    "    "      --     100/0  "    68.3  14                            81                                                                               "    "    "    Al(NO.sub.3).sub.3                                                                    Mg(NO.sub.3).sub.2                                                                    50/50  "    67.2  21                            82                                                                               "    "    "    Al(OH).sub.3                                                                          MgCO.sub.3                                                                            "      "    59.8  18                            __________________________________________________________________________

                                      TABLE 16                                     __________________________________________________________________________     Composition                                                                                      Al.sub.2 O.sub.3 --MgO                                                                     Bending                                                                              Acid                                          ZrO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     CeO.sub.2                                                                           Al.sub.2 O.sub.3 /MgO                                                                 Amount                                                                              Strength                                                                             Resistance                                                                           Transformation                       No (mol %)                                                                             (mol %)                                                                             (mol %)                                                                             (molar ratio)                                                                         (wt %)                                                                              (Kgf/mm.sup.2)                                                                       (mg/cm.sup.2)                                                                        Rate (%)                             __________________________________________________________________________     1  97.0 3.0  0    40/60  2.0  95.3  3.2   --                                   2  "    "    "    70/30  "    98.2  3.4   --                                   3  "    "    "    90/10  "    99.4  3.4   --                                   4  95.0 "    2.0  40/60  "    87.2  --    10                                   5  "    "    "    70/30  "    92.1  --     7                                   6  "    "    "    90/10  "    92.5  --     8                                   7  97.0 "    0    "      "    107.5 2.7   --                                   8  "    "    "    "      "    115.7 2.0   --                                   9  95.0 "    2.0  "      "    125.3 --    14                                   10 "    "    "    "      "    121.5 --    17                                   __________________________________________________________________________ 

What is claimed is:
 1. A pressureless sintered zirconia ceramic composition consisting essentially of a compound of ZrO₂ consisting essentially of 0.5-5.0 mol% Y₂ O₃ as a stabilizer and 1-30 wt% aluminum and magnesium contents to the amount of said compound of ZrO₂ in terms of Al₂ O₃ and MgO,wherein a molar ratio between the aluminum and magnesium contents is selected from one of the following ranges in terms of Al₂ O₃ and MgO:(a) 35-45/65-55 (b) 60-75/40-25 (c) 85-99/15-1.
 2. The pressureless sintered zirconia ceramic composition of claim 1, wherein said compound of ZrO₂ consists essentially of 1.5-5 mol% Y₂ O₃ as a stabilizer.
 3. The pressureless sintered zirconia ceramic composition of claim 1, wherein said compound of ZrO₂ consists essentially of 1-5 wt% aluminum and magnesium contents to the amount of said compound of ZrO₂ in terms of Al₂ O₃ and MgO.
 4. A pressureless sintered zirconia composition consisting essentially of a compound of ZrO₂ consisting essentially of 0.5-5 mol% Y₂ O₃ and 0.5-12 mol% CeO₂ as stabilizers and 1-30 wt% aluminum and magnesium contents to the amount of said compound of ZrO₂ in terms of Al₂ O₃ and MgO, the total amount of Y₂ O₃ and CeO₂ being 1.0-15 mol%,wherein a molar ratio between the aluminum and magnesium contents is selected from one of the following ranges in terms of Al₂ O₃ and MgO:(a) 35-45/65-55 (b) 60-75/40-25 (c) 85-99/15-1.
 5. The pressureless sintered zirconia ceramic composition of claim 4, wherein said compound of ZrO₂ consists essentially of 1-5 wt% aluminum and magnesium contents to the amount of said compound of ZrO₂ in terms of Al₂ O₃ and MgO. 